A 3-year-old child with a fever of 40 degrees is usually caused by gastroenteritis, pneumonia, meningitis, non-infectious fever and other conditions. 1. Gastroenteritis: a 3-year-old child with a fever of 40 degrees can be caused by gastroenteritis due to eating unclean food, etc. Usually, the child will not only have a fever, but also often accompanied by diarrhea, abdominal pain, nausea, vomiting and other symptoms. 2. Pneumonia: Pneumonia due to bacteria, viruses and other causes may also lead to a fever of 40 degrees in 3-week-old children, pneumonia children usually have fever, cough, shortness of breath, difficulty and other symptoms. 3. Meningitis: 3-week-old children with a fever of 40 degrees may also be due to meningitis, patients usually show fever, headache, vomiting, and even convulsions. 4. non-infectious fever: leukemia, systemic lupus erythematosus, Kawasaki disease, etc. can make the 3-week-old child fever 40 degrees. There may be other reasons for a 3 year old child to have a fever of 40 degrees, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.